Preflight Condition Variables

That depends. Default task sequence variables are populated by the task sequence engine. Default UI++ task sequence variables are populated by the DefaultValues action: uiplusplus.configmgrftw.com/docs/actionconfig/defaultvalues.html. Other variables can be populated in many different ways. The Preflight action can make use of any task sequence variable regardless of what populates its value.

The Preflight action documentation discusses most of this: uiplusplus.configmgrftw.com/docs/actionconfig/preflight.html